KR20150005413A - Methods for Oxide Based Solid Electrolyte - Google Patents

Abstract

An oxide-based solid electrolyte according to the present invention can be Li_x-yLa_3M_2O_12-y. The oxide-based solid electrolyte can further include a doping element. The manufacturing method of the oxide-based solid electrolyte according to the concept of the present invention comprises a step of preparing a precursor including lanthanoid complex and metallic complex; a step of manufacturing an intermediate by using hydrothermal synthesis conducted to the precursor solution; a step of manufacturing a mixture by adding lithium compound and a dopant precursor to the intermediate; and a step of crystalizing the mixture. The oxide-based solid electrolyte manufactured according to the present invention can show high ionic conductivity.

Description

METHOD FOR MANUFACTURING OXIDE-BASED SOLID ELECTROLYTE

The present invention relates to a lithium battery, and more particularly, to the production of an oxide-based solid electrolyte through a hydrothermal reaction.

As energy storage and conversion technologies become more important, interest in lithium batteries is increasing. The lithium battery may include an anode, a separator, a cathode, and electrolytes. The electrolyte serves as a medium through which ions can move between the anode and the cathode. Lithium batteries are very high in energy density compared to other batteries and can be made compact and lightweight, so they have been actively researched and developed as power sources for portable electronic devices and the like. Recently, as the performance of portable electronic devices has improved, the power consumed in portable electronic devices has been increasing. Lithium batteries are required to generate high power. Accordingly, lithium battery electrolytes are required to have high ion conductivity and low electric conductivity.

The lithium battery electrolyte may include an organic liquid electrolyte and an inorganic solid electrolyte. Organic liquid electrolytes are widely used because of their high solubility in lithium salts and their high ionic conductivity and stable electrochemical properties. However, organic liquid electrolytes suffer from many problems related to safety due to their high flammability, volatility, and leakage problems. A lithium battery including an inorganic solid electrolyte can have freedom of battery design. In addition, the inorganic solid electrolyte is prevented from ignition, explosion and the like due to the decomposition reaction of the electrolytic solution and the like, and can have excellent stability.

The present invention relates to a process for preparing an oxide-based solid electrolyte having high purity and ionic conductivity.

The problems to be solved by the present invention are not limited to the above-mentioned problems, and other problems not mentioned can be clearly understood by those skilled in the art from the following description.

The present invention relates to a process for producing an oxide-based solid electrolyte. A method for preparing an oxide-based solid electrolyte according to the concept of the present invention comprises: preparing a precursor including a lanthanide complex and a metal complex; Preparing an intermediate using the hydrothermal synthesis reaction performed on the precursor solution; Adding a lithium compound and a dopant precursor to the intermediate to prepare a mixture; And crystallizing the mixture.

According to one embodiment, the hydrothermal synthesis reaction of the precursor solution may be conducted at 120 ° C to 240 ° C for 2 hours to 48 hours.

According to one embodiment, preparing the precursor solution comprises adding the lanthanide complex and the metal complex to an acidic aqueous solution to produce a precursor solution; And adding a mineralizing agent to the precursor solution to form a precursor precipitate.

According to one embodiment, crystallizing the mixture comprises performing a first crystallization process on the mixture to produce the first oxide-based solid electrolyte; And performing a second crystallization process on the first oxide-based solid electrolyte to produce a second oxide-based solid electrolyte, wherein the second oxide-based solid electrolyte has the same stoichiometric Composition, and may have a different crystal structure.

According to one embodiment, the second oxide-based solid electrolyte may have a higher ionic conductivity than the first oxide-based solid electrolyte.

According to one embodiment, the first oxide-based solid electrolyte may have a tetragonal phase and the second oxide-based solid electrolyte may have a cubic phase.

According to one embodiment, the second oxide-based solid electrolyte may have the formula Li _{x - y} La ₃ M ₂ O _{12 - y} . (Where x is 5 or 7, y is 0.3 to 0.7, and M is any one selected from tantalum, niobium, zirconium, or combinations thereof).

According to one embodiment, the first crystallization process may be performed at 700 ° C to 900 ° C for 6 hours to 12 hours.

According to one embodiment, the second crystallization process may be performed at 1000 ° C to 1100 ° C for 6 hours to 12 hours.

 According to one embodiment, the dopant precursor may comprise at least one selected from aluminum, germanium, silicon gallium, indium, tin, and antimony.

The oxide-based solid electrolyte according to the present invention may include a doping element. Thus, the ionic conductivity of the oxide-based solid electrolyte can be increased. By the hydrothermal synthesis reaction, the lanthanide element can be mixed well with the metal element. Therefore, in the oxide-based solid electrolyte produced through the hydrothermal synthesis reaction, the lithium element, the lanthanide element, the metal element, and the oxygen element can be uniformly distributed in the oxide-based solid electrolyte. The oxide-based solid electrolyte crystallization processes proceed at a relatively low temperature, so that an oxide-based solid electrolyte can be easily produced. Due to the first and second crystallization process conditions of the present invention, the deficiency of the lithium element in the oxide-based solid electrolyte can be prevented. By controlling the stoichiometric ratio of the added lithium element, deficiency of the lithium element in the oxide based solid electrolyte of the present invention can be further prevented. The oxide-based solid electrolyte produced by the present invention can exhibit high purity and high ionic conductivity.

BRIEF DESCRIPTION OF THE DRAWINGS For a more complete understanding and assistance of the invention, reference is made to the following description, taken together with the accompanying drawings,
1 is a flowchart illustrating a method of manufacturing an oxide-based solid electrolyte according to an embodiment of the present invention.
2 is a result of evaluating the ionic conductivities of the experimental examples and the comparative examples.

In order to fully understand the structure and effects of the present invention, preferred embodiments of the present invention will be described with reference to the accompanying drawings. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. It will be apparent to those skilled in the art that the present invention may be embodied in many other specific forms without departing from the spirit or essential characteristics thereof. Those of ordinary skill in the art will understand that the concepts of the present invention may be practiced in any suitable environment.

The terminology used herein is for the purpose of illustrating embodiments and is not intended to be limiting of the present invention. In the present specification, the singular form includes plural forms unless otherwise specified in the specification. As used herein, the terms 'comprises' and / or 'comprising' mean that the stated element, step, operation and / or element does not imply the presence of one or more other elements, steps, operations and / Or additions.

When a film (or layer) is referred to herein as being on another film (or layer) or substrate it may be formed directly on another film (or layer) or substrate, or a third film Or layer) may be interposed.

 Although the terms first, second, third, etc. have been used in various embodiments herein to describe various regions, films (or layers), etc., it is to be understood that these regions, do. These terms are merely used to distinguish any given region or film (or layer) from another region or film (or layer). Thus, the membrane referred to as the first membrane in one embodiment may be referred to as the second membrane in another embodiment. Each embodiment described and exemplified herein also includes its complementary embodiment. Like numbers refer to like elements throughout the specification.

The terms used in the embodiments of the present invention may be construed as commonly known to those skilled in the art unless otherwise defined.

1 is a flowchart illustrating a method of manufacturing an oxide-based solid electrolyte according to an embodiment of the present invention.

Referring to FIG. 1, a precursor solution can be prepared by mixing a metal complex and a lanthanide complex. (S10) For example, a metal complex may be added to the acidic aqueous solution. The metal complex may be a zirconium complex, a tantalum complex, or a niobium complex. In one example, the metal complex may be zirconium dinitrate oxide hydrate. A 1 M aqueous solution of nitric acid (HNO ₃ ) can be used as an acidic aqueous solution. To form the metal complex solution, the metal complex and the acidic aqueous solution may be mixed. For example, the metal complex and the acidic aqueous solution may be mixed by stirring at a temperature of 50 캜 to 60 캜. The metal complex solution may be transparent. Lanthanide complexes may be added to the metal complex solution. As an example, lanthanum nitrate may be used as a lanthanide complex. At this time, the metal complex to lanthanide complex may have a molar ratio of 3: 2. By mixing the lanthanide complex and the metal complex solution, a precursor solution can be prepared. For example, the lanthanide complex and the metal complex solution may be mixed by stirring at a temperature of 50 ° C to 60 ° C. The precursor solution can be transparent. The precursor solution may include lanthanide cations and metal cations. As an example, the precursor solution may include La ⁺ ions and Zr ^{+ 4} ions.

The mineralizer may be added to the precursor solution. For example, sodium hydroxide solution and / or ammonium hydroxide solution may be used as the mineralizer. Optical agents include, for negative ions, for example, in the precursor solution, hydroxide ions (OH ^-) can be supplied. The pH of the precursor solution can be adjusted to approximately 8 to 9. Lanthanide cations (for example, La ⁺³ ) and metal cations (for example, Zr ^{+ 4} ) can bond with the anions (OH ^- ) supplied by the mineralizing agent. Thus, precursor precipitation can be produced from the precursor solution. As an example, the precursor precipitation may include lanthanide hydroxides (e.g., Lanthanum hydroxide, La (OH) ₃ ) and metal hydroxides (e.g., Zirconium hydroxide, M (OH) _y ).

The hydrothermal synthesis reaction can be carried out using the precursor precipitation. (S20) The hydrothermal reaction may mean a synthesis reaction of a substance in an aqueous solution state at high temperature and high pressure. The hydrothermal synthesis reaction may be carried out at a temperature of about 140 ° C to 250 ° C. At a temperature condition lower than several tens of degrees Celsius, the first intermediate may not be formed. When the hydrothermal synthesis reaction proceeds at a temperature higher than 250 ° C, the size or shape of the synthesized first intermediate may be uneven. The hydrothermal synthesis reaction may be carried out for about 8 to 36 hours. If the hydrothermal synthesis reaction proceeds less than 8 hours, the resulting first intermediate may have an excessively small or non-uniform particle size. If the hydrothermal synthesis reaction proceeds longer than 36 hours, the resulting first intermediate may have an excessively large or uneven lip size. The first intermediate may be washed. At this time, high purity alcohol or distilled water may be used. The washed first intermediate may be dried. The drying process may be carried out at a temperature of 100 ° C to 120 ° C using an oven. The intermediate may have a relatively small and uniform size. For example, the intermediate may have a diameter of 15 nm to 20 nm. The intermediate may have a length of 180 nm to 200 nm. In the case of the intermediate prepared by the hydrothermal synthesis reaction, the lanthanide element can be uniformly mixed with the metal element.

To form a mixture, a lithium compound and a dopant precursor may be added to the intermediate. (S30) Before the addition of the lithium compound and the dopant precursor, the first intermediate may be heat treated. The heat treatment of the first intermediate may be performed at 300 to 400 degrees. By the heat treatment of the first intermediate, the impurities contained in the first intermediate can be removed. Accordingly, the produced oxide-based solid electrolyte can have high purity. The lithium compound may be any one selected from lithium acetate, lithium nitrate, lithium chloride, lithium hydroxide, and lithium oxide. For example, the lithium compound may be Li ₂ CO ₃ , LiOH, or LiNO ₃ , more specifically Li ₂ CO ₃ . In order to prevent the inclusion of impurities, the lithium compound may not contain a metal other than lithium. The dopant precursor may be a salt containing a doping element. The dopant precursor may be acetate, nitrate, chloride, hydroxide, or oxide. For example, the doping element of the doping precursor may comprise at least one selected from aluminum, germanium, silicon gallium, indium, tin, and antimony. The doping element may have the same or similar atomic diameter as the lithium element. A lithium compound and a dopant precursor may be mixed with an intermediate to prepare a mixture. The mixing process can be carried out by mechanical mixing, for example, ball milling. At this time, isopropyl alcohol may be added to the intermediate so that a more homogeneous mixture is produced. In the mixture, the lithium element, the lanthanide element, the metal element, and the oxygen element may be uniformly distributed.

The first oxide-based solid electrolyte may be formed by the first crystallization process of the mixture. (S40) The first crystallization process may include heat treatment of the mixture. By the first crystallization step, the lithium element is diffused between the lanthanide element and the metal element to form the first oxide-based solid electrolyte. The first oxide-based solid electrolyte may have a tetragonal phase. The stoichiometric composition and crystallinity of the first oxide-based solid electrolyte can be determined by the first crystallization process conditions. The first crystallization process may be performed at a temperature of 700 deg. C to 900 deg. When the first crystallization process is conducted at a temperature lower than 700 ° C, the first oxide-based solid electrolyte having a tetragonal phase may not be formed. If the first crystallization process is conducted at a temperature condition higher than 900 ° C, the lithium element may be excessively depleted in the first oxide-based solid electrolyte. The first crystallization process may be carried out for 6 to 12 hours. If the first crystallization process is shorter than 6 hours, the first oxide-based solid electrolyte may have poor crystallinity. If the first crystallization process is longer than 12 hours, the amount of lithium element lost in the heat treatment process may be increased. Accordingly, the first oxide-based solid electrolyte can be produced with a low yield. The first oxide-based solid electrolyte of the present invention may have the chemical formula of Li _{x - y} La ₃ M ₂ O _{12 -y} . Wherein x is 5 or 7 and y is 0.3 to 0.7 and M is any one selected from tantalum, Ta, niobium, Nb, zirconium, Zr, and combinations thereof. The lithium element, the lanthanide element, the metal element, and the oxygen element can be uniformly distributed in the first oxide-based solid electrolyte. By the first crystallization process, the doping element can be replaced with the lithium element contained in the first oxide-based solid electrolyte. The more the doping element has a size similar to that of the lithium element, the more easily the lithium element can be substituted with the doping element. The first oxide-based solid electrolyte containing the doping element may have a higher ionic conductivity.

The second oxide-based solid electrolyte may have the same stoichiometric composition as the first oxide-doped solid electrolyte. (S40) The second oxide-based solid electrolyte may have a stoichiometric composition similar to that of the first oxide- And may have a different crystal structure. For example, the second oxide-based solid electrolyte may have a cubic phase. The second oxide-based solid electrolyte may have a higher ionic conductivity than the first oxide-based solid electrolyte. For example, the second oxide-based solid electrolyte may have an ionic conductivity of at least 10 ² times that of the first oxide-based solid electrolyte. The second crystallization process may be performed at a temperature of 1000 ° C to 1100 ° C. When the second crystallization process is conducted at a temperature lower than 1000 ° C, the second oxide-based solid electrolyte having cubic structure may not be formed. When the second crystallization process proceeds at a temperature condition higher than 1100 ° C, the second oxide-based solid electrolyte can be produced at a low yield. For example, the content of impurities in the second oxide-based solid electrolyte may increase, or the lithium element may be excessively deficient. The first crystallization process may be carried out for 6 to 12 hours. If the second crystallization process is shorter than 6 hours, the crystallinity of the second solid electrolyte may be poor. During the second heat treatment process longer than 12 hours, the amount of the lithium element lost in the heat treatment process increases, and the yield of the second oxide based solid electrolyte may be low. In the first and second crystallization steps, the lithium element can be volatilized. Accordingly, in the above-described step S30, the lithium compound may be added in an amount of about 10% by stoichiometrically more than the lithium element contained in the oxide-based solid electrolyte. The second oxide based solid electrolyte produced through the first and second crystallization processes can be further prevented from deficiency of lithium element than when the first crystallization step is omitted. As another example, the shape of the first oxide-based solid electrolyte can be adjusted before the second crystallization process is performed. For example, the first oxide-based solid electrolyte may be made of pellets. As the first oxide-based solid electrolyte is produced in a solid powder state, the shape or size of the first oxide-based solid electrolyte pellet can be easily controlled. The shape or size of the first oxide-based solid electrolyte may be adjusted to suit the application of the lithium battery.

The second oxide based solid electrolyte produced according to the present invention may have the formula Li _{x - y} La ₃ M ₂ O _{12 - y} . Wherein x is 5 or 7 and y is 0.3 to 0.7 and M is any one selected from tantalum, Ta, niobium, Nb, zirconium, Zr, and combinations thereof. The second oxide-based solid electrolyte may include a doping element substituted in the lithium element site. The doping element may be approximately 0.00001 wt% to 5 wt% of the oxide based solid electrolyte. The second oxide-based solid electrolyte containing the doping element may exhibit a high ionic conductivity.

Hereinafter, the production of oxide-based solid electrolytes according to the present invention and the evaluation results of characteristics of the oxide-based solid electrolytes will be described in more detail with reference to the experimental examples of the present invention.

Preparation of oxide-based solid electrolyte

<Experimental Example 1-1>

(Preparation of Intermediate by Hydrothermal Synthesis)

Zirconium dinitrate oxide hydrate and Lanthanum nitrate were added to a 1 M aqueous solution of nitric acid (HNO ₃ ) at a molar ratio of 3: 2. Zirconium dinitrate oxide hydrate and Lanthanum nitrate are in powder form. A stirring process using a hot plate was performed in the aqueous solution until the solution became transparent. Thus, a precursor solution is prepared. An aqueous sodium hydroxide solution and an aqueous ammonia solution were added to the precursor solution until the precursor solution showed a pH of 8 to 9. At this time, a precipitate formed in the precursor solution. The precursor solution was heat-treated at 200 ° C for 20 hours, and the hydrothermal synthesis reaction of the precursor solution proceeded. The synthesized intermediate was washed using high purity alcohol and distilled water. The washed intermediate was dried at 100 &lt; 0 &gt; C. LiCO ₃ was added to the intermediate so that Li: La: Zr had a molar ratio of 7.7: 3: 2. Aluminum was added to the LiCO ₃ added intermediate to form the second intermediate. The mass of aluminum added is 1 wt% of the second intermediate.

(Preparation of Solid Electrolyte)

The second intermediate was heat-treated at 900 占 폚 for 6 hours. The prepared first oxide-based solid electrolyte was pulverized. A pulverized first oxide based solid electrolyte is provided in the mold. The mold was pressurized to prepare the first oxide-based solid electrolyte in the form of pellets. The first oxide-based solid electrolyte pellet has a diameter of 10 mm and a thickness of 2 mm. The first oxide-based solid electrolyte pellet was heat-treated at 1100 DEG C for 12 hours to prepare a second oxide-based solid electrolyte in the form of a pellet.

<Experimental Example 1-2>

The same procedure as in Experimental Example 1-1 was carried out to prepare a second oxide-based solid electrolyte in the form of a pellet. However, in this experiment, germanium was used instead of aluminum.

<Experimental Example 1-3>

The same procedure as in Experimental Example 1-1 was carried out to prepare a second oxide-based solid electrolyte in the form of a pellet. However, in this example, tin was used instead of aluminum.

&Lt; Comparative Example 1-1 > Production of oxide-based solid electrolyte by solid-phase method

(Preparation of Intermediate)

Zirconium dinitrate oxide hydrate, Lanthanum nitrate, and LiCO ₃ are prepared so that Li: La: Zr has a molar ratio of 7.7: 3: 2. The zirconium dinitrate oxide hydrate, Lanthanum nitrate, and LiCO ₃ were mixed by a ball mill method. The zirconium dinitrate oxide hydrate, Lanthanum nitrate, and LiCO _{3 used} are each in powder form.

 (Preparation of Solid Electrolyte)

The intermediate was heat-treated at a temperature of 900 占 폚 for 6 hours to form the first product. The first product was pulverized. The pulverized first product was heat-treated at a temperature of 1100 캜 for 12 hours to prepare a second product. The second product was pulverized. The pulverized second product was produced in the same manner as described in Experimental Example 1, and the second product in the form of pellets was produced. The second product in the form of a pellet has the same size as that of Experimental Example 1 (for example, 10 mm in diameter and 2 mm in thickness). The second product in the form of pellets was heat-treated at 1200 DEG C for 24 hours to prepare an oxide-based solid electrolyte.

&Lt; Comparative Example 1-2 > Preparation of undoped oxide-based solid electrolyte

The same processes as in Experimental Example 1-1 were carried out to prepare a second oxide-based solid electrolyte in the form of a pellet. However, aluminum is not added in this comparative example.

Characterization of Solid Electrolyte

<Experimental Example 2-1>

Copper was coated on both sides of the second oxide-based solid electrolyte pellet prepared in Experimental Example 1-1 to a thickness of 6 mu m to prepare a cell. The cell was subjected to an AC impedance in the range of 10 ^-1 to 10 ⁵ Hz to measure the electrical conductivity of the second oxide-based solid electrolyte. The electrical conductivity of the second oxide-based solid electrolyte was measured using a frequency response analyzer (Solartron HF 1225).

<Experimental Example 2-2>

The ion conductivity of the solid electrolyte of Experimental Example 1-2 was calculated by performing the same process as Experimental Example 2-1. As described above, the solid electrolyte of Experimental Example 1-2 contains germanium as a doping element.

<Experimental Example 2-3>

The ionic conductivity of the solid electrolyte of Experimental Example 1-3 was calculated by the same method as Experimental Example 2-1. As described above, the solid electrolyte of Experimental Example 1-3 contains tin as a doping element.

&Lt; Comparative Example 2-1 >

The ion conductivity of the solid electrolyte of Comparative Example 1-1 was calculated in the same manner as in Experimental Example 2-1. As described above, the solid electrolyte of Comparative Example 1-1 was prepared by the solid phase method.

&Lt; Comparative Example 2-2 &

The ionic conductivity of the solid electrolyte of Comparative Example 1-2 was calculated in the same manner as in Experimental Example 2-1. As described above, the solid electrolyte of Comparative Example 1-2 does not contain a doping element.

2 is a result of evaluating the ionic conductivities of the experimental examples and the comparative examples.

Referring to FIG. 2, it can be confirmed that the experimental examples (e1, e2, e3) show higher ion conductivity than the comparative example 2-1 (c1). When the solid electrolyte is produced by the solid-phase method, the lanthanide element, the metal element, and the lithium element may not be uniformly mixed in the intermediate. Accordingly, the produced oxide-based solid electrolyte may have a low purity. In addition, the production of the oxide-based solid electrolyte by the solid phase method may include a heat treatment process at 1100 ° C or higher, for example, 1200 ° C. At this temperature condition, the lithium element may be deficient in the oxide-based solid electrolyte. At this temperature condition, an impurity (for example, La ₂ Zr ₂ O ₇ ) may be formed. The solid electrolyte in the experimental examples can be produced by hydrothermal synthesis. In the case of the intermediate prepared by the hydrothermal synthesis method, the lanthanide element can be uniformly mixed with the metal element. Accordingly, in the case of the oxide-based solid electrolyte of the experimental examples, the lithium element, the lanthanide element, the metal element, and the oxygen element can be uniformly distributed. In addition, the oxide-based solid electrolyte produced by the hydrothermal synthesis reaction may have a small and uniform size. The solid electrolytes of Examples (e1, e2, e3) can be produced by the hydrothermal synthesis process, and thus can exhibit high ion conductivity.

It can be seen that the experimental examples (e1, e2, e3) show higher ionic conductivity than Comparative Example 2-2 (c2). The oxide-based solid electrolyte of Comparative Example 2-2 (c2) may not contain a doping element. The oxide-based solid electrolyte of Experimental Examples (e1, e2, e3) can further improve the ionic conductivity by further including the doping element.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the present invention is not limited to the disclosed exemplary embodiments, and various changes and modifications may be made by those skilled in the art without departing from the scope and spirit of the invention. Change is possible.

Claims (10)

Preparing a precursor comprising a lanthanide complex and a metal complex;
Preparing an intermediate using the hydrothermal synthesis reaction performed on the precursor solution;
Adding a lithium compound and a dopant precursor to the intermediate to prepare a mixture; And
And crystallizing the mixture. &Lt; RTI ID = 0.0 &gt; 11. &lt; / RTI &gt;
The method according to claim 1,
The hydrothermal synthesis reaction of the precursor solution
Lt; RTI ID = 0.0 &gt; 240 C &lt; / RTI &gt; for 2 to 48 hours.
The method according to claim 1,
Preparing the precursor solution comprises:
Adding the lanthanide complex and the metal complex to an acidic aqueous solution to prepare a precursor solution; And
And adding a mineralizer to the precursor solution to form a precursor precipitate.
The method according to claim 1,
Crystallizing the mixture comprises:
Subjecting the mixture to a first crystallization process to produce the first oxide-based solid electrolyte; And
And performing a second crystallization process on the first oxide-based solid electrolyte to produce a second oxide-based solid electrolyte,
Wherein the second oxide-based solid electrolyte has the same stoichiometric composition as the first oxide-based solid electrolyte, and has a different crystal structure.
5. The method of claim 4,
Wherein the second oxide-based solid electrolyte has a higher ion conductivity than the first oxide-based solid electrolyte.
5. The method of claim 4,
The first oxide-based solid electrolyte has a tetragonal phase,
And the second oxide-based solid electrolyte has a cubic phase.
5. The method of claim 4,
Wherein the second oxide based solid electrolyte has the formula Li _{x - y} La ₃ M ₂ O _{12 - y} .
(Where x is 5 or 7, y is 0.3 to 0.7, and M is any one selected from tantalum, niobium, zirconium, or combinations thereof).
The method of claim 4, wherein
The first crystallization step
Wherein the reaction is carried out at 700 ° C to 900 ° C for 6 hours to 12 hours.
5. The method of claim 4,
The second crystallization step
Wherein the reaction is carried out at 1000 ° C to 1100 ° C for 6 hours to 12 hours.
The method according to claim 1,
Wherein the dopant precursor comprises at least one selected from aluminum, germanium, silicon gallium, indium, tin, and antimony.

Images